1 /* 2 * Copyright (C) 2016 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package com.android.server; 18 19 import android.util.ArrayMap; 20 import android.util.ArraySet; 21 import android.util.Slog; 22 23 import java.io.FileDescriptor; 24 import java.io.PrintWriter; 25 26 /** 27 * LockGuard is a mechanism to help detect lock inversions inside the system 28 * server. It works by requiring each lock acquisition site to follow this 29 * pattern: 30 * 31 * <pre> 32 * synchronized (LockGuard.guard(lock)) { 33 * } 34 * </pre> 35 * 36 * <pre> 37 * $ find services/ -name "*.java" -exec sed -i -r \ 38 * 's/synchronized.?\((.+?)\)/synchronized \(com.android.server.LockGuard.guard\(\1\)\)/' {} \; 39 * </pre> 40 * 41 * The {@link #guard(Object)} method internally verifies that all locking is 42 * done in a consistent order, and will log if any inversion is detected. For 43 * example, if the calling thread is trying to acquire the 44 * {@code ActivityManager} lock while holding the {@code PackageManager} lock, 45 * it will yell. 46 * <p> 47 * This class requires no prior knowledge of locks or their ordering; it derives 48 * all of this data at runtime. However, this means the overhead is 49 * <em>substantial</em> and it should not be enabled by default. For example, 50 * here are some benchmarked timings: 51 * <ul> 52 * <li>An unguarded synchronized block takes 40ns. 53 * <li>A guarded synchronized block takes 50ns when disabled. 54 * <li>A guarded synchronized block takes 460ns per lock checked when enabled. 55 * </ul> 56 */ 57 public class LockGuard { 58 private static final String TAG = "LockGuard"; 59 60 private static ArrayMap<Object, LockInfo> sKnown = new ArrayMap<>(0, true); 61 62 private static class LockInfo { 63 /** Friendly label to describe this lock */ 64 public String label; 65 66 /** Child locks that can be acquired while this lock is already held */ 67 public ArraySet<Object> children = new ArraySet<>(0, true); 68 } 69 findOrCreateLockInfo(Object lock)70 private static LockInfo findOrCreateLockInfo(Object lock) { 71 LockInfo info = sKnown.get(lock); 72 if (info == null) { 73 info = new LockInfo(); 74 info.label = "0x" + Integer.toHexString(System.identityHashCode(lock)) + " [" 75 + new Throwable().getStackTrace()[2].toString() + "]"; 76 sKnown.put(lock, info); 77 } 78 return info; 79 } 80 81 /** 82 * Check if the calling thread is holding any locks in an inverted order. 83 * 84 * @param lock The lock the calling thread is attempting to acquire. 85 */ guard(Object lock)86 public static Object guard(Object lock) { 87 // If we already hold this lock, ignore 88 if (lock == null || Thread.holdsLock(lock)) return lock; 89 90 // Check to see if we're already holding any child locks 91 boolean triggered = false; 92 final LockInfo info = findOrCreateLockInfo(lock); 93 for (int i = 0; i < info.children.size(); i++) { 94 final Object child = info.children.valueAt(i); 95 if (child == null) continue; 96 97 if (Thread.holdsLock(child)) { 98 Slog.w(TAG, "Calling thread " + Thread.currentThread().getName() + " is holding " 99 + lockToString(child) + " while trying to acquire " 100 + lockToString(lock), new Throwable()); 101 triggered = true; 102 } 103 } 104 105 if (!triggered) { 106 // If no trouble found above, record this lock as being a valid 107 // child of all locks currently being held 108 for (int i = 0; i < sKnown.size(); i++) { 109 final Object test = sKnown.keyAt(i); 110 if (test == null || test == lock) continue; 111 112 if (Thread.holdsLock(test)) { 113 sKnown.valueAt(i).children.add(lock); 114 } 115 } 116 } 117 118 return lock; 119 } 120 121 /** 122 * Report the given lock with a well-known label. 123 */ installLock(Object lock, String label)124 public static void installLock(Object lock, String label) { 125 final LockInfo info = findOrCreateLockInfo(lock); 126 info.label = label; 127 } 128 lockToString(Object lock)129 private static String lockToString(Object lock) { 130 final LockInfo info = sKnown.get(lock); 131 if (info != null) { 132 return info.label; 133 } else { 134 return "0x" + Integer.toHexString(System.identityHashCode(lock)); 135 } 136 } 137 dump(FileDescriptor fd, PrintWriter pw, String[] args)138 public static void dump(FileDescriptor fd, PrintWriter pw, String[] args) { 139 for (int i = 0; i < sKnown.size(); i++) { 140 final Object lock = sKnown.keyAt(i); 141 final LockInfo info = sKnown.valueAt(i); 142 pw.println("Lock " + lockToString(lock) + ":"); 143 for (int j = 0; j < info.children.size(); j++) { 144 pw.println(" Child " + lockToString(info.children.valueAt(j))); 145 } 146 pw.println(); 147 } 148 } 149 } 150